The present invention relates to a polyester film for magnetic recording media and, particularly, to a polyester film for magnetic recording media which is excellent in adhesiveness, light-shielding property, durability and electromagnetic transducing property. More in detail, the present invention relates to a biaxially oriented polyester film for magnetic recording media, containing uniformly carbon black particles in the film and having the following specific properties: ##EQU2## wherein T.sub.900 represents a light transmittance of the film at a wavelength of 900 nm, Ra represents an average surface roughness of the film, nl represents the number of protuberances of 0.81 to 1.08 .mu.m in height existing on the surface of the film per film area of 25 cm.sup.2, and n.sub.2 represents the number of protuberances of more than 1.08 .mu.m in height existing on the surface of the film per film area of 25 cm.sup.2.
Polyester films have hitherto been widely used as base films for magnetic recording media such as floppy disks and magnetic tapes because of their excellent strength and chemical resistance, etc. With the rapid progress in the recording density of magnetic recording media, a base film for such recording media, in particular, floppy disks has recently been strongly demanded to have a thinner magnetic layer. Most of floppy disks and magnetic tapes are generally mechanically positioned by optically detecting index holes or reading tapes, so that they are required to have an excellent light-shielding property. However, if a magnetic layer is made thinner in the case where a transparent polyester film is used as a base film, the light-shielding property is disadvantageously lowered together with the reduction in thickness of the magnetic layer. On the other hand, if the amount of carbon black in the magnetic layer is increased in order to maintain the light-shielding property, the durability is disadvantageously lowered. Thus, achievement of a thinner magnetic layer has been difficult.
To solve these problems, a method of uniformly dispersing carbon black particles in a film, thereby both enhancing the adhesiveness between the film and the magnetic layer and endowing a high light-shielding property to the base film itself has conventionally been tried. Carbon black particles, however, have strong cohesiveness, and the aggregates are apt to form coarse protuberances on the surface of the film, so that it is difficult to obtain a high electromagnetic transducing property. This attempt has, therefore, been abandoned because it cannot attain the intended object of producing a high-density magnetic recording media. As a countermeasure, a three-layered film composed of a carbon black layer having a light-shielding property and a layer having no light-shielding property provided on both sides of the carbon black layer, has been proposed. A method of producing such a three-layered film, however, has the disadvantages that it necessitates a novel equipment such as a co-extruding device; the use as the starting material of a recycled film is difficult, resulting in rise in the cost; and mass production is difficult.
Therefore, the offering of a single-layer base film, not a multi-layer base film, for magnetic recording media which has excellent adhesiveness, light-shielding property and durability, and which is capable of endowing a high electromagnetic transducing property has become necessary.
As a result of researches on the above-described problems, it has been found by the present inventors that by finely dispersing carbon black in the film and restricting the light transmittance of the film, its average surface roughness, and the number of coarse protuberances of a specific height existing on the surface of the film within a specific range the produced base film for magnetic recording media has excellent adhesiveness, light-shielding property and durability, and is capable of endowing a high electromagnetic transducing property in the form of a single layer.